Welcome to the site leojr. Your vehicle isn't non-typical here, we have plenty of truck drivers.
I have to say that increasing speed is VERY rarely ever going to get you better mileage. If we look at every
speed vs mpg chart we have, it backs up that (in top gear) lower speeds equals better mileage. There is only one exception that I've ever seen and the mpg gain was very minimal.
In any case, aeromods are your friend to increasing mileage at any speed faster than walking. I'd look into them.